Plasma-assisted gas production

ABSTRACT

Methods and apparatus are provided for plasma-assisted gas production. In one embodiment, a gas, which includes at least one atomic or molecular species, can flow into a cavity ( 305 ). The gas can be subjected to electromagnetic radiation having a frequency less than about 333 GHz (optionally in the presence of a plasma catalyst) such that a plasma ( 310 ) forms in the cavity ( 305 ). A filter ( 315 ) capable of passing the atomic or molecular species, but preventing others from passing, can be in fluid communication with the cavity ( 305 ). In this way, the selected species can be extracted and collected, for storage or immediate use.

CROSS-REFERENCE OF RELATED APPLICATIONS

Priority is claimed to U.S. Provisional Patent Application No.60/378,693, filed May 8, 2002, No. 60/430,677, filed Dec. 4, 2002, andNo. 60/435,278, filed Dec. 23, 2002, all of which are fully incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for plasma-assisted gasproduction, including hydrogen production.

BACKGROUND OF THE INVENTION

It is known that plasmas can be used to produce gas. However, suchmethods can be limited by a number of factors, including the conditionsrequired to ignite, modulate, and sustain plasmas and the rates at whichgases can be produced. Moreover, some conventional methods and apparatususe vacuum equipment to ignite the plasma, however the use of suchequipment can limit gas production flexibility.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION

A plasma-assisted gas production method may be provided. The method caninclude (1) flowing a gas into a cavity, wherein the gas includes atleast a first species, (2) subjecting the gas to electromagneticradiation having a frequency less than about 333 GHz such that a plasmaforms in the cavity, wherein the cavity is in fluid communication withat least one filter, (3) extracting the first species through thefilter, and (4) collecting the first species.

A plasma-assisted gas production apparatus may also be provided. Theapparatus can include (1) a cavity configured such that a plasma canform therein by subjecting a gas to radiation having a frequency lessthan about 333 GHz, (2) at least one filter associated with the cavityhaving a plasma-facing side and a plasma-opposing side, wherein thefilter is configured to selectively permit a first species, andsubstantially prevents other species, which may be present in theplasma, to pass through the filter, (3) a gas source connected to thecavity for supplying the gas to the cavity, (4) a radiation sourceconnected to the cavity for supplying the radiation to the cavity, and(5) a collection device in communication with the plasma-opposing sideof the filter.

A plasma catalyst for initiating, modulating, and sustaining a plasmamay further be provided. The catalyst can be passive or active. Apassive plasma catalyst can include any object capable of inducing aplasma by deforming a local electric field (e.g., an electromagneticfield) consistent with this invention, without necessarily addingadditional energy. An active plasma catalyst, on the other hand, is anyparticle or high energy wave packet capable of transferring a sufficientamount of energy to a gaseous atom or molecule to remove at least oneelectron from the gaseous atom or molecule in the presence ofelectromagnetic radiation. In both cases, a plasma catalyst can improve,or relax, the environmental conditions required to ignite a plasma.

Additional plasma catalysts, and methods and apparatus for igniting,modulating, and sustaining a plasma for producing a gas consistent withthis invention are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the invention will be apparent upon consideration ofthe following detailed description, taken in conjunction with theaccompanying drawings, in which like reference characters refer to likeparts throughout, and in which:

FIG. 1 shows a schematic diagram of an illustrative plasma-assisted gasproduction system consistent with this invention;

FIG. 1A shows an illustrative embodiment of a portion of aplasma-assisted gas production system for adding a powder plasmacatalyst to a plasma cavity for igniting, modulating, or sustaining aplasma in a cavity consistent with this invention;

FIG. 2 shows an illustrative plasma catalyst fiber with at least onecomponent having a concentration gradient along its length consistentwith this invention;

FIG. 3 shows an illustrative plasma catalyst fiber with multiplecomponents at a ratio that varies along its length consistent with thisinvention;

FIG. 4 shows another illustrative plasma catalyst fiber that includes acore under layer and a coating consistent with this invention;

FIG. 5 shows a cross-sectional view of the plasma catalyst fiber of FIG.4, taken from line 5-5 of FIG. 4 consistent with this invention;

FIG. 6 shows an illustrative embodiment of another portion of a plasmasystem including an elongated plasma catalyst that extends throughignition port consistent with this invention;

FIG. 7 shows an illustrative embodiment of an elongated plasma catalystthat can be used in the system of FIG. 6 consistent with this invention;

FIG. 8 shows another illustrative embodiment of an elongated plasmacatalyst that can be used in the system of FIG. 6 consistent with thisinvention;

FIG. 9 shows an illustrative embodiment of a portion of aplasma-assisted gas production system for directing ionizing radiationinto a radiation chamber consistent with this invention;

FIG. 10 shows a simplified cross-sectional view of an illustrativeapparatus for plasma-assisted gas production consistent with thisinvention;

FIG. 11 shows a simplified cross-sectional view of another illustrativegas production apparatus consistent with this invention;

FIG. 12 shows a simplified cross-sectional view of still anotherillustrative gas production apparatus consistent with this invention;

FIG. 13 shows a cross-sectional view of an illustrative coaxial cable(similar to a coaxial waveguide), taken along line 13-13 of FIG. 10,consistent with this invention; and

FIG. 14 shows a flow chart of an illustrative gas production methodconsistent with this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

This invention relates to methods and apparatus for plasma-assisted gasproduction, and can be used to lower energy costs and increase gasproduction efficiency and flexibility.

The following commonly owned, concurrently filed U.S. patentapplications are hereby incorporated by reference in their entireties:U.S. patent application Ser. No. 10/513,221, U.S. patent applicationSer. No. 10/13,393, PCT Application US03/14132, now expired, U.S. patentapplication Ser. No. 10/513,394, U.S. patent application Ser. No.10/513,305, U.S. patent application Ser. No. 10/513,607, U.S. patentapplication Ser. No. 10/449,600, PCT Application US03/14034, nowexpired, U.S. patent application Ser. No. 10/430,416, U.S. patentapplication Ser. No. 10/430,415, PCT Application US03/14133, nowexpired, U.S. patent application Ser. No. 10/513,606, U.S. patentapplication Ser. No. 10/513,309, PCT Application US03/14122, U.S. patentapplication Ser. No. 10/513,397, U.S. patent application Ser. No.10/513,605, PCT Application US03/14137, now expired, U.S. patentapplication Ser. No. 10/430,426, PCT Application US03/14121, nowexpired, U.S. patent application Ser. No. 10/513,604, and PCTApplication US03/14135, now expired.

Illustrative Plasma System

FIG. 1 shows illustrative plasma system 10 consistent with one aspect ofthis invention. In this embodiment, cavity 12 is formed in a vessel thatis positioned inside radiation chamber (i.e., applicator) 14. In anotherembodiment (not shown), the vessel 12 and radiation chamber 14 are thesame, thereby eliminating the need for two separate components. Thevessel in which cavity 12 is formed can include one or moreradiation-transmissive insulating layers to improve its thermalinsulation properties without significantly shielding cavity 12 from theradiation. As described more fully below, system 10 can be used toproduce gas consistent with this invention.

In one embodiment, cavity 12 is formed in a vessel made of ceramic. Dueto the extremely high temperatures that can be achieved with plasmasconsistent with this invention, a ceramic capable of operating at about3,000 degrees Fahrenheit can be used. The ceramic material can include,by weight, 29.8% silica, 68.2% alumina, 0.4% ferric oxide, 1% titania,0.1% lime, 0.1% magnesia, 0.4% alkalies, which is sold under Model No.LW-30 by New Castle Refractories Company, of New Castle, Pa. It will beappreciated by those of ordinary skill in the art, however, that othermaterials, such as quartz, and those different from the one describedabove, can also be used consistent with the invention.

In one successful experiment, a plasma was formed in a partially opencavity inside a first brick and topped with a second brick. The cavityhad dimensions of about 2 inches by about 2 inches by about 1.5 inches.At least two holes were also provided in the brick in communication withthe cavity: one for viewing the plasma and at least one hole forproviding the gas. The size of the cavity can depend on the desiredplasma process being performed. Also, the cavity can at least beconfigured to prevent the plasma from rising/floating away from theprimary gas production region (e.g., the surface of a filter).

Cavity 12 can be connected to one or more gas sources 24 (e.g., a sourceof argon, nitrogen, xenon, krypton, hydrocarbon, or anotherhydrogen-containing gas) by line 20 and control valve 22, which may bepowered by power supply 28. Line 20 may be tubing (e.g., between about1/16 inch and about ¼ inch, such as about ⅛″). Also, if desired, avacuum pump can be connected to the chamber to remove any fumes that maybe generated during plasma processing.

A radiation leak detector (not shown) was installed near source 26 andwaveguide 30 and connected to a safety interlock system to automaticallyturn off the radiation (e.g., microwave) power supply if a leak above apredefined safety limit, such as one specified by the FCC and/or OSHA(e.g., 5 mW/cm²), was detected.

Radiation source 26, which may be powered by electrical power supply 28,can direct radiation energy into chamber 14 through one or morewaveguides 30. It will be appreciated by those of ordinary skill in theart that source 26 can be connected directly to cavity 12 or chamber 14,thereby eliminating waveguide 30. The radiation energy entering cavity12 is used to ignite a plasma within the cavity. This plasma can besubstantially sustained and confined to the cavity by couplingadditional radiation with the catalyst.

Radiation energy can be supplied through circulator 32 and tuner 34(e.g., 3-stub tuner). Tuner 34 can be used to minimize the reflectedpower as a function of changing ignition or processing conditions,especially before the plasma has formed because microwave power, forexample, will be strongly absorbed by the plasma.

As explained more fully below, the location of radiation-transmissivecavity 12 in chamber 14 may not be critical if chamber 14 supportsmultiple modes, and especially when the modes are continually orperiodically mixed. As also explained more fully below, motor 36 can beconnected to mode-mixer 38 for making the time-averaged radiation energydistribution substantially uniform throughout chamber 14. Furthermore,window 40 (e.g., a quartz window) can be disposed in one wall of chamber14 adjacent to cavity 12, permitting temperature sensor 42 (e.g., anoptical pyrometer) to be used to view a process inside cavity 12. In oneembodiment, the optical pyrometer output can increase from zero volts asthe temperature rises to within the tracking range.

Sensor 42 can develop output signals as a function of the temperature orany other monitorable condition associated with a work piece (not shown)within cavity 12 and provide the signals to controller 44. Dualtemperature sensing and heating, as well as automated cooling rate andgas flow controls can also be used. Controller 44 in turn can be used tocontrol operation of power supply 28, which can have one outputconnected to source 26 as described above and another output connectedto valve 22 to control gas flow into cavity 12.

The invention may be practiced, for example, with a microwave source at2.45 GHz provided by Communications and flower Industries (CPI),although radiation having any frequency less than about 333 GHz can beused. The 2.45 GHz system provided continuously variable microwave powerfrom about 0.5 kilowatts to about 5.0 kilowatts. A 3-stub tuner allowedimpedance matching for maximum power transfer and a dual directionalcoupler (not shown) was used to measure forward and reflected powers.Also, optical pyrometers were used for remote sensing of the sampletemperature.

As mentioned above, radiation having any frequency less than about 333GHz can be used consistent with this invention. For example,frequencies, such as power line frequencies (about 50 Hz to about 60Hz), can be used, although the pressure of the gas from which the plasmais formed may be lowered to assist with plasma ignition. Also, any radiofrequency or microwave frequency can be used consistent with thisinvention, including frequencies greater than about 100 kHz. In mostcases, the gas pressure for such relatively high frequencies need not belowered to ignite, modulate, or sustain a plasma, thereby enabling manyplasma-assisted, gas production methods to occur at atmosphericpressures and above.

The equipment was be computer controlled using LabView 6i software,which provided real-time temperature monitoring and microwave powercontrol. Noise was reduced by using sliding averages of suitable numberof data points. Also, to improve speed and computational efficiency, thenumber of stored data points in the buffer array were limited by usingshift-registers and buffer-sizing. The pyrometer measured thetemperature of a sensitive area of about 1 cm², which was used tocalculate an average temperature. The pyrometer sensed radiantintensities at two wavelengths and fit those intensities using Planck'slaw to determine the temperature. It will be appreciated, however, thatother devices and methods for monitoring and controlling temperature arealso available and can be used consistent with this invention. Forexample, control software that can be used consistent with thisinvention is described in commonly owned, concurrently filed Kumar etal., PCT Application US03/14135, now expired which is herebyincorporated by reference in its entirety.

Chamber 14 had several glass-covered viewing ports with radiationshields and one quartz window for pyrometer access. Several ports forconnection to a vacuum pump and a gas source were also provided,although not necessarily used.

System 10 also included a closed-loop deionized water cooling system(not shown) with an external heat exchanger cooled by tap water. Duringoperation, the deionized water first cooled the magnetron, then theload-ump in the circulator (used to protect the magnetron), and finallythe radiation chamber through water channels welded on the outer surfaceof the chamber.

Plasma Catalysts

A plasma catalyst consistent with this invention can include one or moredifferent materials and may be either passive or active. A plasmacatalyst can be used, among other things, to ignite, modulate, and/orsustain a plasma at a gas pressure that is less than, equal to, orgreater than atmospheric pressure.

One method of forming a plasma consistent with this invention caninclude subjecting a gas in a cavity to electromagnetic radiation havinga frequency less than about 333 GHz in the presence of a passive plasmacatalyst. A passive plasma catalyst consistent with this invention caninclude any object capable of inducing a plasma by deforming a localelectric field (e.g., an electromagnetic field) consistent with thisinvention, without necessarily adding additional energy through thecatalyst, such as by applying an electric voltage to create a spark.

A passive plasma catalyst consistent with this invention can also be anano-particle or a nano-tube. As used herein, the term “nano-particle”can include any particle having a maximum physical dimension less thanabout 100 nm that is at least electrically semi-conductive. Also, bothsingle-walled and multi-walled carbon nanotubes, doped and undoped, canbe particularly effective for igniting plasmas consistent with thisinvention because of their exceptional electrical conductivity andelongated shape. The nanotubes can have any convenient length and can bea powder fixed to a substrate. If fixed, the nanotubes can be orientedrandomly on the surface of the substrate or fixed to the substrate(e.g., at some predetermined orientation) while the plasma is ignited orsustained.

A passive plasma catalyst can also be a powder consistent with thisinvention, and need not comprise nano-particles or nano-tubes. It can beformed, for example, from fibers, dust particles, flakes, sheets, etc.When in powder form, the catalyst can be suspended, at leasttemporarily, in a gas. By suspending the powder in the gas, the powdercan be quickly dispersed throughout the cavity and more easily consumed,if desired.

In one embodiment, the powder catalyst can be carried into the cavityand at least temporarily suspended with a carrier gas. The carrier gascan be the same or different from the gas that forms the plasma. Also,the powder can be added to the gas prior to being introduced to thecavity. For example, as shown in FIG. 1A, radiation source 52 can supplyradiation to radiation cavity 55, in which plasma cavity 60 is placed.Powder source 65 can provide catalytic powder 70 into gas stream 75. Inan alternative embodiment, powder 70 can be first added to cavity 60 inbulk (e.g., in a pile) and then distributed in the cavity in any numberof ways, including flowing a gas through or over the bulk powder. Inaddition, the powder can be added to the gas for igniting, modulating,or sustaining a plasma by moving, conveying, drizzling, sprinkling,blowing, or otherwise, feeding the powder into or within the cavity.

In one experiment, a plasma was ignited in a cavity by placing a pile ofcarbon fiber powder in a copper pipe that extended into the cavity.Although sufficient radiation was directed into the cavity, the copperpipe shielded the powder from the radiation and no plasma ignition tookplace. However, once a carrier gas began flowing through the pipe,forcing the powder out of the pipe and into the cavity, and therebysubjecting the powder to the radiation, a plasma was nearlyinstantaneously ignited in the cavity.

A powder plasma catalyst consistent with this invention can besubstantially non-combustible, thus it need not contain oxygen or burnin the presence of oxygen. Thus, as mentioned above, the catalyst caninclude a metal, carbon, a carbon-based alloy, a carbon-based composite,an electrically conductive polymer, a conductive silicone elastomer, apolymer nanocomposite, an organic-inorganic composite, and anycombination thereof.

Also, powder catalysts can be substantially uniformly distributed in theplasma cavity (e.g., when suspended in a gas), and plasma ignition canbe precisely controlled within the cavity. Uniform ignition can beimportant in certain applications, including those applicationsrequiring brief plasma exposures, such as in the form of one or morebursts. Still, a certain amount of time can be required for a powdercatalyst to distribute itself throughout a cavity, especially incomplicated, multi-chamber cavities. Therefore, consistent with anotheraspect of this invention, a powder catalyst can be introduced into thecavity through a plurality of ignition ports to more rapidly obtain amore uniform catalyst distribution therein (see below).

In addition to powder, a passive plasma catalyst consistent with thisinvention can include, for example, one or more microscopic ormacroscopic fibers, sheets, needles, threads, strands, filaments, yarns,twines, shavings, slivers, chips, woven fabrics, tape, whiskers, or anycombination thereof. In these cases, the plasma catalyst can have atleast one portion with one physical dimension substantially larger thananother physical dimension. For example, the ratio between at least twoorthogonal dimensions can be at least about 1:2, but could be greaterthan about 1:5, or even greater than about 1:10.

Thus, a passive plasma catalyst can include at least one portion ofmaterial that is relatively thin compared to its length. A bundle ofcatalysts (e.g., fibers) may also be used and can include, for example,a section of graphite tape. In one experiment, a section of tape havingapproximately thirty thousand strands of graphite fiber, each about 2-3microns in diameter, was successfully used. The number of fibers in andthe length of a bundle are not critical to igniting, modulating, orsustaining the plasma. For example, satisfactory results have beenobtained using a section of graphite tape about one-quarter inch long.One type of carbon fiber that has been successfully used consistent withthis invention is sold under the trademark Magnamite®, Model No.AS4C-GP3K, by the Hexcel Corporation, of Anderson, S.C. Also,silicon-carbide fibers have been successfully used.

A passive plasma catalyst consistent with another aspect of thisinvention can include one or more portions that are, for example,substantially spherical, annular, pyramidal, cubic, planar, cylindrical,rectangular or elongated.

The passive plasma catalysts discussed above can include at least onematerial that is at least electrically semi-conductive. In oneembodiment, the material can be highly conductive. For example, apassive plasma catalyst consistent with this invention can include ametal, an inorganic material, carbon, a carbon-based alloy, acarbon-based composite, an electrically conductive polymer, a conductivesilicone elastomer, a polymer nanocomposite, an organic-inorganiccomposite, or any combination thereof. Some of the possible inorganicmaterials that can be included in the plasma catalyst include carbon,silicon carbide, molybdenum, platinum, tantalum, tungsten, carbonnitride, and aluminum, although other electrically conductive inorganicmaterials may work just as well.

In addition to one or more electrically conductive materials, a passiveplasma catalyst consistent with this invention can include one or moreadditives (which need not be electrically conductive). As used herein,the additive can include any material that a user wishes to add to theplasma. Therefore, the catalyst can include the additive itself, or itcan include a precursor material that, upon decomposition, can form theadditive. Thus, the plasma catalyst can include one or more additivesand one or more electrically conductive materials in any desirableratio, depending on the ultimate desired composition of the plasma andthe process using the plasma.

The ratio of the electrically conductive components to the additives ina passive plasma catalyst can vary over time while being consumed. Forexample, during ignition, the plasma catalyst could desirably include arelatively large percentage of electrically conductive components toimprove the ignition conditions. On the other hand, if used whilesustaining the plasma, the catalyst could include a relatively largepercentage of additives. It will be appreciated by those of ordinaryskill in the art that the component ratio of the plasma-catalyst used toignite and sustain the plasma could be the same.

A predetermined ratio profile can be used to simplify many plasmaprocesses. In many conventional plasma processes, the components withinthe plasma are added as necessary, but such addition normally requiresprogrammable equipment to add the components according to apredetermined schedule. However, consistent with this invention, theratio of components in the catalyst can be varied, and thus the ratio ofcomponents in the plasma itself can be automatically varied. That is,the ratio of components in the plasma at any particular time can dependon which of the catalyst portions is currently being consumed by theplasma. Thus, the catalyst component ratio can be different at differentlocations within the catalyst. And, the current ratio of components in aplasma can depend on the portions of the catalyst currently and/orpreviously consumed, especially when the flow rate of a gas passingthrough the plasma chamber is relatively slow.

A passive plasma catalyst consistent with this invention can behomogeneous, inhomogeneous, or graded. Also, the plasma catalystcomponent ratio can vary continuously or discontinuously throughout thecatalyst. For example, in FIG. 2, the ratio can vary smoothly forming agradient along a length of catalyst 100. Catalyst 100 can include astrand of material that includes a relatively low concentration of acomponent at section 105 and a continuously increasing concentrationtoward section 110.

Alternatively, as shown in FIG. 3, the ratio can vary discontinuously ineach portion of catalyst 120, which includes, for example, alternatingsections 125 and 130 having different concentrations. It will beappreciated that catalyst 120 can have more than two section types.Thus, the catalytic component ratio being consumed by the plasma canvary in any predetermined fashion. In one embodiment, when the plasma ismonitored and a particular additive is detected, further processing canbe automatically commenced or terminated.

Another way to vary the ratio of components in a sustained plasma is byintroducing multiple catalysts having different component ratios atdifferent times or different rates. For example, multiple catalysts canbe introduced at approximately the same location or at differentlocations within the cavity. When introduced at different locations, theplasma formed in the cavity can have a component concentration gradientdetermined by the locations of the various catalysts. Thus, an automatedsystem can include a device by which a consumable plasma catalyst ismechanically inserted before and/or during plasma igniting, modulating,and/or sustaining.

A passive plasma catalyst consistent with this invention can also becoated. In one embodiment, a catalyst can include a substantiallynon-electrically conductive coating deposited on the surface of asubstantially electrically conductive material. Alternatively, thecatalyst can include a substantially electrically conductive coatingdeposited on the surface of a substantially electrically non-conductivematerial. FIGS. 4 and 5, for example, show fiber 140, which includesunderlayer 145 and coating 150. In one embodiment, a plasma catalystincluding a carbon core is coated with nickel to prevent oxidation ofthe carbon.

A single plasma catalyst can also include multiple coatings. If thecoatings are consumed during contact with the plasma, the coatings couldbe introduced into the plasma sequentially, from the outer coating tothe innermost coating, thereby creating a time-release mechanism. Thus,a coated plasma catalyst can include any number of materials, as long asa portion of the catalyst is at least electrically semi-conductive.

Consistent with another embodiment of this invention, a plasma catalystcan be located entirely within a radiation cavity to substantiallyreduce or prevent radiation energy leakage. In this way, the plasmacatalyst does not electrically or magnetically couple with the vesselcontaining the cavity or to any electrically conductive object outsidethe cavity. This prevents sparking at the ignition port and preventsradiation from leaking outside the cavity during the ignition andpossibly later if the plasma is sustained. In one embodiment, thecatalyst can be located at a tip of a substantially electricallynon-conductive extender that extends through an ignition port.

FIG. 6, for example, shows radiation chamber 160 in which plasma cavity165 is placed. Plasma catalyst 170 is elongated and extends throughignition port 175. As shown in FIG. 7, and consistent with thisinvention, catalyst 170 can include electrically conductive distalportion 180 (which is placed in chamber 160) and electricallynon-conductive portion 185 (which is placed substantially outsidechamber 160, but can extend somewhat into chamber 160). Thisconfiguration can prevent an electrical connection (e.g., sparking)between distal portion 180 and chamber 160.

In another embodiment, shown in FIG. 8, the catalyst can be formed froma plurality of electrically conductive segments 190 separated by andmechanically connected to a plurality of electrically non-conductivesegments 195. In this embodiment, the catalyst can extend through theignition port between a point inside the cavity and another pointoutside the cavity, but the electrically discontinuous profilesignificantly prevents sparking and energy leakage.

Another method of forming a plasma consistent with this inventionincludes subjecting a gas in a cavity to electromagnetic radiationhaving a frequency less than about 333 GHz in the presence of an activeplasma catalyst, which generates or includes at least one ionizingparticle.

An active plasma catalyst consistent with this invention can be anyparticle or high energy wave packet capable of transferring a sufficientamount of energy to a gaseous atom or molecule to remove at least oneelectron from the gaseous atom or molecule in the presence ofelectromagnetic radiation. Depending on the source, the ionizingparticles can be directed into the cavity in the form of a focused orcollimated beam, or they may be sprayed, spewed, sputtered, or otherwiseintroduced.

For example, FIG. 9 shows radiation source 200 directing radiation intoradiation chamber 205. Plasma cavity 210 is positioned inside of chamber205 and may permit a gas to flow therethrough via ports 215 and 216.Source 220 can direct ionizing particles 225 into cavity 210. Source 220can be protected, for example, by a metallic screen which allows theionizing particles to pass through but shields source 220 fromradiation. If necessary, source 220 can be water-cooled.

Examples of ionizing particles consistent with this invention caninclude x-ray particles, gamma ray particles, alpha particles, betaparticles, neutrons, protons, and any combination thereof. Thus, anionizing particle catalyst can be charged (e.g., an ion from an ionsource) or uncharged and can be the product of a radioactive fissionprocess. In one embodiment, the vessel in which the plasma cavity isformed could be entirely or partially transmissive to the ionizingparticle catalyst. Thus, when a radioactive fission source is locatedoutside the cavity, the source can direct the fission products throughthe vessel to ignite the plasma. The radioactive fission source can belocated inside the radiation chamber to substantially prevent thefission products (i.e., the ionizing particle catalyst) from creating asafety hazard.

In another embodiment, the ionizing particle can be a free electron, butit need not be emitted in a radioactive decay process. For example, theelectron can be introduced into the cavity by energizing the electronsource (such as a metal), such that the electrons have sufficient energyto escape from the source. The electron source can be located inside thecavity, adjacent the cavity, or even in the cavity wall. It will beappreciated by those of ordinary skill in the art that the anycombination of electron sources is possible. A common way to produceelectrons is to heat a metal, and these electrons can be furtheraccelerated by applying an electric field.

In addition to electrons, free energetic protons can also be used tocatalyze a plasma. In one embodiment, a free proton can be generated byionizing hydrogen and, optionally, accelerated with an electric field.

Multi-Mode Radiation Cavities

A radiation waveguide, cavity, or chamber can be designed to support orfacilitate propagation of at least one electromagnetic radiation mode.As used herein, the term “mode” refers to a particular pattern of anystanding or propagating electromagnetic wave that satisfies Maxwell'sequations and the applicable boundary conditions (e.g., of the cavity).In a waveguide or cavity, the mode can be any one of the variouspossible patterns of propagating or standing electromagnetic fields.Each mode is characterized by its frequency and polarization of theelectric field and/or the magnetic field vectors. The electromagneticfield pattern of a mode depends on the frequency, refractive indices ordielectric constants, and waveguide or cavity geometry.

A transverse electric (TE) mode is one whose electric field vector isnormal to the direction of propagation. Similarly, a transverse magnetic(TM) mode is one whose magnetic field vector is normal to the directionof propagation. A transverse electric and magnetic (TEM) mode is onewhose electric and magnetic field vectors are both normal to thedirection of propagation. A hollow metallic waveguide does not typicallysupport a normal TEM mode of radiation propagation. Even thoughradiation appears to travel along the length of a waveguide, it may doso only by reflecting off the inner walls of the waveguide at someangle. Hence, depending upon the propagation mode, the radiation (e.g.,microwave) may have either some electric field component or somemagnetic field component along the axis of the waveguide (often referredto as the z-axis).

The actual field distribution inside a cavity or waveguide is asuperposition of the modes therein. Each of the modes can be identifiedwith one or more subscripts (e.g., TE₁₀ (“tee ee one zero”). Thesubscripts normally specify how many “half waves” at the guidewavelength are contained in the x and y directions. It will beappreciated by those skilled in the art that the guide wavelength can bedifferent from the free space wavelength because radiation propagatesinside the waveguide by reflecting at some angle from the inner walls ofthe waveguide. In some cases, a third subscript can be added to definethe number of half waves in the standing wave pattern along the z-axis.

For a given radiation frequency, the size of the waveguide can beselected to be small enough so that it can support a single propagationmode. In such a case, the system is called a single-mode system (i.e., asingle-mode applicator). The TE₁₀ mode is usually dominant in arectangular single-mode waveguide. As the size of the waveguide (or thecavity to which the waveguide is connected) increases, the waveguide orapplicator can sometimes support additional higher order modes forming amulti-mode system. When many modes are capable of being supportedsimultaneously, the system is often referred to as highly moded.

A simple, single-mode system has a field distribution that includes atleast one maximum and/or minimum. The magnitude of a maximum largelydepends on the amount of radiation supplied to the system. Thus, thefield distribution of a single mode system is strongly varying andsubstantially non-uniform.

Unlike a single-mode cavity, a multi-mode cavity can support severalpropagation modes simultaneously, which, when superimposed, results in acomplex field distribution pattern. In such a pattern, the fields tendto spatially smear and, thus, the field distribution usually does notshow the same types of strong minima and maxima field values within thecavity. In addition, as explained more fully below, a mode-mixer can beused to “stir” or “redistribute” modes (e.g., by mechanical movement ofa radiation reflector). This redistribution desirably provides a moreuniform time-averaged field distribution within the cavity.

A multi-mode cavity consistent with this invention can support at leasttwo modes, and may support many more than two modes. Each mode has amaximum electric field vector. Although there may be two or more modes,one mode may be dominant and has a maximum electric field vectormagnitude that is larger than the other modes. As used herein, amufti-mode cavity may be any cavity in which the ratio between the firstand second mode magnitudes is less than about 1:10, or less than about1:5, or even less than about 1:2. It will be appreciated by those ofordinary skill in the art that the smaller the ratio, the moredistributed the electric field energy between the modes, and hence themore distributed the radiation energy is in the cavity.

The distribution of plasma within a plasma cavity may strongly depend onthe distribution of the applied radiation. For example, in a pure singlemode system, there may only be a single location at which the electricfield is a maximum. Therefore, a strong plasma may only form at thatsingle location. In many applications, such a strongly localized plasmacould undesirably lead to non-uniform plasma treatment or heating (i.e.,localized overheating and underheating).

Whether or not a single or multi-mode cavity is used consistent withthis invention, it will be appreciated by those of ordinary skill in theart that the cavity in which the plasma is formed can be completelyclosed or partially open. For example, in certain applications, such asin plasma-assisted furnaces, the cavity could be entirely closed. See,for example, commonly owned, concurrently filed Kumar et al. PCTApplication US03/14133, now expired, which is fully incorporated hereinby reference. In other applications, however, it may be desirable toflow a gas through the cavity, and therefore the cavity must be open tosome degree. In this way, the flow, type, and pressure of the flowinggas can be varied over time. This may be desirable because certain gasesthat facilitate plasma formation, such as argon, are easier to ignitebut may not be needed during subsequent plasma processing.

Mode-Mixing

For many plasma-assisted applications, a cavity containing a uniformplasma is desirable. However, because microwave radiation can have arelatively long wavelength (e.g., several tens of centimeters),obtaining a uniform distribution can be difficult to achieve. As aresult, consistent with one aspect of this invention, the radiationmodes in a multi-mode cavity can be mixed, or redistributed, over aperiod of time. Because the field distribution within the cavity mustsatisfy all of the boundary conditions set by the inner surface of thecavity (e.g., if metallic), those field distributions can be changed bychanging the position of any portion of that inner surface.

In one embodiment consistent with this invention, a movable reflectivesurface can be located inside the radiation cavity. The shape and motionof the reflective surface should, when combined, change the innersurface of the cavity during motion. For example, an “L” shaped metallicobject (i.e., “mode-mixer”) when rotated about any axis will change thelocation or the orientation of the reflective surfaces in the cavity andtherefore change the radiation distribution therein. Any otherasymmetrically shaped object can also be used (when rotated), butsymmetrically shaped objects can also work, as long as the relativemotion (e.g., rotation, translation, or a combination of both) causessome change in the location or orientation of the reflective surfaces.In one embodiment, a mode-mixer can be a cylinder that is rotable aboutan axis that is not the cylinder's longitudinal axis.

Each mode of a multi-mode cavity may have at least one maximum electricfield vector, but each of these vectors could occur periodically acrossthe inner dimension of the cavity. Normally, these maxima are fixed,assuming that the frequency of the radiation does not change. However,by moving a mode-mixer such that it interacts with the radiation, it ispossible to move the positions of the maxima. For example, mode-mixer 38can be used to optimize the field distribution within cavity 14 suchthat the plasma ignition conditions and/or the plasma sustainingconditions are optimized. Thus, once a plasma is excited, the positionof the mode-mixer can be changed to move the position of the maxima fora uniform time-averaged plasma process (e.g., heating).

Thus, consistent with this invention, mode-mixing can be useful duringplasma ignition. For example, when an electrically conductive fiber isused as a plasma catalyst, it is known that the fiber's orientation canstrongly affect the minimum plasma-ignition conditions. It has beenreported, for example, that when such a fiber is oriented at an anglethat is greater than 60° to the electric field, the catalyst does littleto improve, or relax, these conditions. By moving a reflective surfaceeither in or near the cavity, however, the electric field distributioncan be significantly changed.

Mode-mixing can also be achieved by launching the radiation into theapplicator chamber through, for example, a rotating waveguide joint thatcan be mounted inside the applicator chamber. The rotary joint can bemechanically moved (e.g., rotated) to effectively launch the radiationin different directions in the radiation chamber. As a result, achanging field pattern can be generated inside the applicator chamber.

Mode-mixing can also be achieved by launching radiation in the radiationchamber through a flexible waveguide. In one embodiment, the waveguidecan be mounted inside the chamber. In another embodiment, the waveguidecan extend into the chamber. The position of the end portion of theflexible waveguide can be continually or periodically moved (e.g., bent)in any suitable manner to launch the radiation (e.g., microwaveradiation) into the chamber at different directions and/or locations.This movement can also result in mode-mixing and facilitate more uniformplasma processing (e.g., heating) on a time-averaged basis.Alternatively, this movement can be used to optimize the location of aplasma for ignition or other plasma-assisted process.

If the flexible waveguide is rectangular, a simple twisting of the openend of the waveguide will rotate the orientation of the electric and themagnetic field vectors in the radiation inside the applicator chamber.Then, a periodic twisting of the waveguide can result in mode-mixing aswell as rotating the electric field, which can be used to assistignition, modulation, or sustaining of a plasma.

Thus, even if the initial orientation of the catalyst is perpendicularto the electric field, the redirection of the electric field vectors canchange the ineffective orientation to a more effective one. Thoseskilled in the art will appreciate that mode-mixing can be continuous,periodic, or preprogrammed.

In addition to plasma ignition, mode-mixing can be useful duringsubsequent plasma processing to reduce or create (e.g., tune) “hotspots” in the chamber. When a microwave cavity only supports a smallnumber of modes (e.g., less than 5), one or more localized electricfield maxima can lead to “hot spots” (e.g., within cavity 12). In oneembodiment, these hot spots could be configured to coincide with one ormore separate, but simultaneous, plasma ignitions or processing events.Thus, the plasma catalyst can be located at one or more of thoseignition or subsequent processing positions.

Multi-Location Ignition

A plasma can be ignited using multiple plasma catalysts at differentlocations. In one embodiment, multiple fibers can be used to ignite theplasma at different points within the cavity. Such multi-point ignitioncan be especially beneficial when a uniform plasma ignition is desired.For example, when a plasma is modulated at a high frequency (i.e., tensof Hertz and higher), or ignited in a relatively large volume, or both,substantially uniform instantaneous striking and restriking of theplasma can be improved. Alternatively, when plasma catalysts are used atmultiple points, they can be used to sequentially ignite a plasma atdifferent locations within a plasma chamber by selectively introducingthe catalyst at those different locations. In this way, a plasmaignition gradient can be controllably formed within the cavity, ifdesired.

Also, in a multi-mode cavity, random distribution of the catalystthroughout multiple locations in the cavity increases the likelihoodthat at least one of the fibers, or any other passive plasma catalystconsistent with this invention, is optimally oriented with the electricfield lines. Still, even where the catalyst is not optimally oriented(not substantially aligned with the electric field lines), the ignitionconditions are improved.

Furthermore, because a catalytic powder can be suspended in a gas, eachpowder particle may have the effect of being placed at a differentphysical location within the cavity, thereby improving ignitionuniformity within the cavity.

Dual-Cavity Plasma Igniting/Sustaining

A dual-cavity arrangement can be used to ignite and sustain a plasmaconsistent with this invention. In one embodiment, a system includes atleast a first ignition cavity and a second cavity in fluid communicationwith the first cavity. To ignite a plasma, a gas in the first ignitioncavity can be subjected to electromagnetic radiation having a frequencyless than about 333 GHz, optionally in the presence of a plasmacatalyst. In this way, the proximity of the first and second cavitiesmay permit a plasma formed in the first cavity to ignite a plasma in thesecond cavity, which may be sustained with additional electromagneticradiation.

In one embodiment of this invention, the first cavity can be very smalland designed primarily, or solely for plasma ignition. In this way, verylittle microwave energy may be required to ignite the plasma, permittingeasier ignition, especially when a plasma catalyst is used consistentwith this invention.

In one embodiment, the first cavity may be a substantially single modecavity and the second cavity is a multi-mode cavity. When the firstignition cavity only supports a single mode, the electric fielddistribution may strongly vary within the cavity, forming one or moreprecisely located electric field maxima. Such maxima are normally thefirst locations at which plasmas ignite, making them ideal points forplacing plasma catalysts. It will be appreciated, however, that when aplasma catalyst is used, it need not be placed in the electric fieldmaximum and, many cases, need not be oriented in any particulardirection.

Plasma-Assisted Gas Production

Methods and apparatus for producing a gas from a plasma are provided.Generally, a plasma, including at least an atomic or molecular species,is formed from a gas. The plasma can be brought into contact with asurface of a selective barrier (e.g., filter), which can selectivelypass the species and substantially prevent others. In one embodiment,for example, the species can be adsorbed onto the surface of thebarrier, pass through the barrier, and be collected on the other side ofthe barrier.

One type of species that can be collected consistent with this inventionis hydrogen. During operation, hydrogen can dissociate on theplasma-facing surface of the filter to produce hydrogen atoms. Then, thehydrogen atoms can combine on the other, low-pressure side of thebarrier and desorb as a hydrogen gas molecule. Once hydrogen moleculesare formed, they can be collected (e.g., for immediate use or storage).

FIG. 10 shows a cross-sectional view of illustrative apparatus 300 forplasma-assisted gas production consistent with this invention. Apparatus300 can include cavity 305 configured such that plasma 310 can formtherein by subjecting a gas to electromagnetic radiation having afrequency less than about 333 GHz. As shown in FIG. 10, the gas can besupplied to cavity 305 by a gas source through one or more ports. Anygas containing the desired species can be used. Moreover, those speciescan be added to the gas separately, and even after the plasma isignited, if desired.

Some of the gas supplied to cavity 305 can also be removed through oneor more different ports. As described more fully below, electromagneticradiation can be supplied to cavity 305 by an electromagnetic radiationsource indirectly through a waveguide or by coupling the source directlyto cavity 305. Also, as shown in FIG. 1, the radiation source can alsobe coupled to an outer chamber, and cavity 305 can be radiationtransmissive.

Apparatus 300 can also include at least one filter 315 associated withcavity 305. Filter 315 can be configured to selectively permit a firstatomic or molecular species (e.g., hydrogen) to pass through it, andsubstantially prevent other species (e.g., carbon), that may also bepresent in plasma 310. The selected species can pass through filter 315from plasma-facing side 317 to plasma-opposing side 319 of filter 315.After the selected species passes through filter 315, any type ofcollection device (not shown) in communication with plasma-opposing side319 can be used to collect the selected species.

Apparatus 300 can also include at least one plasma catalyst in or nearcavity 305. The plasma catalyst can be active or passive consistent withthis invention, but it can also be any other type of plasma ignitiondevice.

Filter 315 can have any shape (e.g., a tube or a plate), as long as oneside is configured to be plasma-facing and another side configured to beplasma-opposing. To maximize gas production, the plasma-facing side canhave a relatively large surface area. Filter 315 can be designed topermit hydrogen, for example, to pass through it. In this case, thefilter can be formed, for example, from a metal hydride, palladium,palladium oxide, ruthenium-palladium, palladium-silver,palladium-copper, uranium, magnesium, titanium, lithium-aluminum,lanthanum-nickel-aluminum, zirconium, ceramic, any combination thereof,or any alloy thereof. In one embodiment, filter 315 can include apalladium tube.

When palladium or a palladium alloy is used to produce hydrogen, thepalladium surface may act as a selective barrier, passing primarilyatomic hydrogen through its wall, while substantially excluding otherspecies. Molecular hydrogen can be adsorbed onto the surface where it isdissociated to become atomic hydrogen. Although the invention is notlimited to any particular theory, hydrogen atoms may share theirelectrons with the palladium metal lattice, permitting the hydrogenatoms to diffuse though the lattice in a direction determined by thepressure gradient. The hydrogen atoms may combine on the low-pressureside of the filter and desorb as a hydrogen molecule.

Other types of filter materials can also be used, depending on the typeof atomic and/or molecular species being extracted. Moreover, filter 315can include multiple filtering layers (not shown). In this way, theselected species can be purified by sequentially passing through eachlayer. Mufti-layer filters consistent with this invention permit thepressure gradient across each filter layer to be smaller than across asingle layer, which may make each layer more selective.

Cavity 305 can have any convenient shape capable of substantiallyconfining a plasma. In one embodiment, as shown in FIG. 10, cavity 305can be formed between inner tube (i.e., filter) 315 and outer tube 350.Outer tube 350 can be electrically conductive to substantially confineradiation, as well as plasma 310. On the other hand, if outer tube 350is made from a radiation transmissive material, such as ceramic orquartz, it will be appreciated that an additional electricallyconductive chamber or shell (such as shown in FIG. 1) can be placedaround tube 350 for safety reasons.

As mentioned above, electromagnetic radiation can be supplied to cavity305 using any type of radiation source in any convenient manner. FIG.10, for example, shows coaxial waveguide 360 configured to directelectromagnetic radiation into cavity 305 via tapered waveguide 365.Radiation-transmissive barrier 362 can permit radiation to enter cavity305 but prevent plasma 310 from passing in the other direction, out ofcavity 305. In an alternative embodiment, coaxial waveguide 360 can beconfigured to direct electromagnetic radiation directly into cavity 305,without the use of a tapered waveguide. The use of tapered waveguide 365permits, however, the use of a larger cavity, which in turn permitsfilters with larger surface areas. Thus, if the outer diameter of innerconductor 361 is not equal to (e.g., less than) the outer diameter offilter 315, a tapered inner connector can be used to connect the twotogether.

In one embodiment, outer tube 350 is electrically conductive andelectrically conductive end plate 398 can move axially (e.g., insidetube 350). On the one hand, if plasma 310 does not sufficiently absorbthe electromagnetic radiation in a single pass, displacement of endplate 398 by a quarter wavelength of the radiation can be used to shiftthe standing wave pattern inside the cavity (reversing the positions ofthe minima and maxima). Consequently, a periodic oscillatory motion ofendplate 398 by a quarter wavelength will smear out the fielddistribution over time and remove the hot spots along filter 315.

On the other hand, if plasma 310 absorbs the radiation strongly, thelength of the plasma region can be selected so that about half of theradiation is absorbed in a single pass. The distance between barrier 364and endplate 398 can now be set (e.g., one quarter, three quarters, etc.of a wavelength) to absorb the remaining power “in phase” when theradiation is reflected back from endplate 398. Those with ordinary skillin the art will appreciate that a similar concept can be applied toother designs (e.g., in the apparatus shown in FIG. 11). In any case,mode-shifting can be continuous, periodic, stepwise, or otherwiseprogrammed.

For example, a larger cavity permits the use of a tubular filter that islarger than the core of the coaxial cable (or waveguide) that can beused to provide the radiation to the cavity. For example, FIG. 11 showsa simplified cross-sectional view of illustrative plasma-assisted gasproduction apparatus 400 consistent with this invention. Like apparatus300, apparatus 400 can include cavity 405, filter 410, coaxial waveguide415, tapered waveguide 420, and radiation-transmissive barrier 425. Asmentioned above, filter 410 can have nearly any shape, but preferablyhas a large surface area to increase the rate at which gas is extractedfrom the plasma. For example, if filter 410 is an elongated tube, it canhave a substantially larger diameter than the core of coaxial cable 417.Also, as shown in FIG. 11, filter 410 need not be connected or otherwiseshaped to conform with core 417. Finally, a filter consistent with thisinvention can include multiple filters, which may be disposed onmultiple tubes or form the tubes themselves.

FIG. 12 shows another simplified cross-sectional view of illustrativegas production apparatus 450 consistent with this invention. Apparatus450 can include cavity 455, filter 460, coaxial waveguide 465, taperedwaveguide 470, and radiation-transmissive barrier 475. Again, filter 460can have nearly any shape, but preferably has a large surface area toincrease the rate at which gas is extracted from the plasma formed incavity 455. As shown in FIG. 12, filter 460 can include multiple filtercomponents to maximize surface area, and those components can beconnected (as shown) or disconnected. If connected, a single gas outlet480 can be used to collect the gas produced by all of the components. Ifdisconnected, multiple gas outlets can be used to separately collect thegas from each of the components (not shown).

FIG. 13 shows a cross-sectional view of illustrative coaxial cable 360,taken along line 13-13 of FIG. 10. As shown in FIG. 13, core 361 has anouter radius designated by the letter “a” and outer shield 363 has aninner radius designated by the letter “b.” When the ratio b:a is betweenabout 2.5:1 and about 3.0:1, the maximum electric field for a givenradiation power that occurs at the outer surface of core 361 can bereduced. When the ratio is about 2.72:1, the maximum electric field atthat surface can be minimized. The same geometry ratio also holds truefor a tubular plasma cavity, which can be defined by an outer radius ofan inner filter tube (e.g., filter 315 of FIG. 10) and an inner radiusof outer tube (e.g., tube 350 of FIG. 10), or by the outer radius of aninner tube and an inner radius of outer filter tube (not shown). Byminimizing the electric field at the filter surface, it is possible tosubstantially prevent arcing and overheating at the outer surface offilter 315, even when a relatively long filter is used with a relativelylarge amount of radiation power.

The tubular coaxial geometry of apparatus 300 permits a TEM mode ofoperation. It will be appreciated, however, that various othergeometries (e.g., rectangular) will permit various other modes ofoperation as well, including single and multi-modes of operation.

Apparatus 300 can also include a voltage source configured to apply abias to a filter. For example, as shown in FIG. 10, voltage source 320can apply an electric bias to filter 315 via coaxial cable 325. In thiscase, inner cable 330 of cable 325 can be electrically connected tofilter 315 via electrically conductive tube 335. Tube 335 can be madefrom any material capable of transporting the atomic or molecularspecies after passing from filter 315, including, for example, steel orcopper. Outer shield 340 of cable 325 can be electrically connected toouter chamber 350, and optionally grounded. Alternatively, voltagesource 320 can apply an electric bias to filter 315 via core 361 ofcoaxial cable 360 when the core is electrically connected to filer 315.Outer shield 363 of cable 360 can also be electrically connected toouter chamber 350, and optionally grounded, if desired.

It will be appreciated that electrical isolator 390<e.g., ceramic orquartz) can be located along steel tube 395 to prevent electrificationof outer tube 350. Also, barrier 364 can be radiation-transmissive oropaque, depending on the design requirements. Like barrier 362, barrier364 can be used to substantially confine plasma 310 in a regionproximate to filter 315. It will be appreciated by those of ordinaryskill in the art, however, that barrier 364 and isolator 390 can beeliminated, especially when voltage source 320 is not used.

FIG. 14 shows an illustrative gas production method consistent with thisinvention. The method can include: (1) flowing a gas into a cavity instep 500, (2) subjecting the gas to electromagnetic radiation having afrequency less than about 333 GHz such that a plasma forms in the cavityin step 505, wherein the cavity is in fluid communication with at leastone filter, (3)-extracting a first species from the plasma through afilter in step 510, and (4) collecting the species in step 515.

In one embodiment consistent with this invention, step 505 can includeigniting the plasma in the cavity by subjecting the gas to the radiationin the presence of at least one plasma catalyst. As explained in detailabove, a plasma catalyst can be active or passive, or it can be anyother device capable of igniting, modulating, or sustaining a plasma.The use of a plasma catalyst consistent with this invention enables themethod of FIG. 14 to be performed below, at, or above atmosphericpressure.

Higher gas pressures can allow for higher plasma pressures, which can beused to increase the rate of extraction and collection. For example,when a pressure gradient is formed across a filter such that theplasma-facing surface (e.g., surface 317 of filter 315) is at a higherpressure than the opposing surface (e.g., surface 319 of filter 315),the selected atomic or molecular species can be extracted at a fasterrate. Thus, the use of a plasma catalyst can be used to ignite plasmasat higher pressures, enabling larger gradients to form without vacuumequipment. The use of vacuum equipment on the side of the opposingsurface can also be used to increase the magnitude of the pressuregradient, especially during ignition.

Another way to increase the extraction rate is to apply an electric biasto the filter, such as shown in FIG. 10. Although the invention is notnecessarily limited to any particular theory, the applied bias can beused to accelerate, and concentrate, the plasma closer the plasma-facingsurface of the filter. Higher concentrations may increase the absorptionrate at that surface, which in turn can increase the rate at which gasis produced on the other side.

The rate of extraction can depend on the temperature of the filter. Ifthe filter extracts faster at higher temperatures, then the temperaturecan be increased by exposure to the plasma. Using appropriatetemperature control, the temperature can be maintained at an optimumtemperature, which may be the highest temperature that the filter canwithstand. As used herein, this temperature is referred to as atransition temperature, which could, for example, be the melting orbreakdown temperature of the filter. When palladium is used, thetemperature can be maintained at about 400 degrees Celsius, but, ingeneral, between about 100 degrees Celsius and about 1,500 degreesCelsius.

The temperature of the filter can be cooled using any conventionalcooling technique, including, for example, flowing a fluid throughchannels embedded or attached to the filter. Another way to cool thefilter is to pass a fluid, such as water, through the center of theaxial filter 315. In this way, the fluid can be used to cool filter 315and simultaneously absorb hydrogen gas that has passed through filter315. The hydrogen gas could then be removed from the fluid in asubsequent step. It will be appreciated that a fluid need not passthrough the core of an axial filter. Rather, the fluid could pass overthe outer surface of the filter or, as mentioned above, through channelsembedded in the filter.

Once gas is being extracted through a filter, it can be collectedconsistent with this invention. Collection can involve, for example,pumping the first species into a gas container. Alternatively,collection can involve exposing the gas to (e.g., an active area of) afuel cell. Such a fuel cell may form part of a residential or industrialpower supply or it may be located on a vehicle, such as an automobile, atrain, a plane, a motorcycle, or any other device that needs mobilepower.

A filter surface can also be cleaned consistent with this invention bysupplying a cleaning gas to the cavity, forming a cleaning plasma withthe cleaning gas, and applying an electric bias to the filter such thatcharged particles forming the plasma are accelerated toward the filtersufficiently to at least partially remove any residue that may havedeposited on the filter.

In the foregoing described embodiments, various features are groupedtogether in a single embodiment for purposes of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby incorporated into this Detailed Description ofEmbodiments, with each claim standing on its own as a separate preferredembodiment of the invention.

1. A plasma assisted gas production method comprising: flowing a gasinto a main cavity, wherein the gas comprises at least a first species;subjecting the gas to electromagnetic radiation having a frequency lessthan about 333 GHz such that a plasma forms in the cavity, and whereinthe cavity is in fluid communication with at least one filter;extracting the first species through the filter; and collecting thefirst species.
 2. The method of claim 1, wherein the first speciescompries hydrogen.
 3. The method of claim 1, wherein the subjectingcomprises igniting the plasma by subjecting the gas in the cavity to theradiation in the presence of at least one plasma catalyst comprising amaterial that is at least electrically semi-conductive.
 4. The method ofclaim 3, wherein the material comprises at least one of metal, inorganicmaterial, carbon, carbon-based alloy, carbon-based composite,electrically conductive polymer, conductive silicone elastomer, polymernanocomposite, and organic-inorganic composite.
 5. The method of claim4, wherein the material is in the form of at least one of anano-particle, a nano-tube, a powder, a dust, a flake, a fiber, a sheet,a needle, a thread, a strand, a filament, a yarn, a twine, a shaving, asilver, a chip, a woven fabric, a tape, and a whisker.
 6. The method ofclaim 5, wherein the plasma catalyst comprises carbon fiber.
 7. Themethod of claim 3, wherein the plasma catalyst comprises a powder. 8.The method of claim 1, wherein the subjecting comprises subjecting thegas to the radiation in the presence of an active plasma catalystcomprising at least one ionizing particle.
 9. The method of claim 8,wherein the at least one ionizing particle comprises a beam ofparticles.
 10. The method of claim 8, wherein the particle is at leastone of an x-ray particle, a gamma ray particle, an alpha particle, abeta particle, a neutron, an electron, an ion, and a proton.
 11. Themethod of claim 8, wherein the ionizing particle comprises a radioactivefission product.
 12. The method of claim 1, wherein the subjectingoccurs at a gas pressure that is at least atmospheric pressure.
 13. Themethod of claim 1, wherein the filter comprises at least one of metalhydride, palladium, palladium oxide, ruthenium-palladium,palladium-silver, palladium-copper, uranium, magnesium, titanium,lithium-aluminum, lanthanum-nickel-aluminum, zirconium, ceramic, anycombination thereof, and any alloy thereof.
 14. The method of claim 1,wherein the filter has a plasma-facing surface and an opposing surface,and wherein the extracting comprises forming a pressure gradient acrossthe filter such that the plasma-facing surface is at a higher pressurethan the opposing surface.
 15. The method of claim 1, wherein theextracting comprises applying an electric bias to the filter.
 16. Themethod of claim 1, wherein the filter has a transition temperature, andwherein the extracting comprises maintaining the temperature of thefilter at a temperature between about 100 degrees Celsius and about 1500degrees Celsius.
 17. The method of claim 16, wherein the extractingcomprises allowing the plasma to contact the filter to heat the filterto a desirable operating temperature.
 18. The method of claim 1, whereinthe filter comprises a plurality of filtering layers, and wherein theextracting comprises passing the first species through each of thefiltering layers to obtain a high purity of the first species.
 19. Themethod of claim 1, wherein the at least one filter comprises a pluralityof filters disposed on a plurality of respective tubes.
 20. The methodof claim 1, wherein the filter is disposed on a surface of a tube, andwherein the extracting comprises causing the first species to passthrough the filter from a location outside the tube to a location insidethe tube.
 21. The method of claim 1, wherein the main cavity is formedbetween an inner filtering tube and a metallic tube, the inner filteringtube comprising the filter and wherein the extracting comprises causingthe first species to pass through the inner tube from a location insidethe main cavity to a location inside the inner tube.
 22. The method ofclaim 21, further comprising directing the electromagnetic radiationinto the main cavity through a coaxial waveguide.
 23. The method ofclaim 21, wherein the inner tube has an outer diameter and the outertube has an inner diameter, wherein the ratio of the inner diameter tothe outer diameter is between about 2.5 and about 3.0.
 24. The method ofclaim 23, wherein the ratio is about 2.72.
 25. The method of claim 21,wherein the radiation comprises a TEM mode, the method furthercomprising shifting the mode axially by a least about a quarter of awavelength.
 26. The method of claim 21, wherein the main cavity has afirst axial end and a second axial end, the method further compriseslaunching the radiation into the main cavity from at least the firstaxial end.
 27. The method of claim 1, wherein the outer tube comprises amaterial that substantially transmits the radiation and the inner tubesubstantially reflects the radiation.
 28. The method of claim 1, furthercomprising applying an electric bias to the filter.
 29. The method ofclaim 1, wherein the first species is in a gaseous from after theextracting, and wherein the collecting comprises pumping the firstspecies into a gas container after the extracting.
 30. The method ofclaim 1, wherein the first species is in a gaseous form after theextracting, and wherein the collecting comprises exposing the firstspecies to a fuel cell.
 31. The method of claim 1, further comprisingcleaning the filter, wherein the cleaning comprises: supplying acleaning gas to the main cavity; forming a cleaning plasma with thecleaning gas, where the cleaning plasma comprises charged particles; andapplying an electric bias to the filter such that the charged particlesare accelerated toward the filter sufficiently to at least partiallyremove a residue deposited on the filter.
 32. The method of claim 1,wherein the collecting comprises flowing a fluid in contact with thefilter such that the species is absorbed by the fluid.
 33. Aplasma-assisted gas production apparatus comprising: a cavity configuredsuch that a plasma can form therein by subjecting a gas toelectromagnetic radiation having a frequency less than about 333 GHz; atleast one filter associated with the cavity having a plasma-facing sideand a plasma-opposing side, wherein the at least one filter isconfigured to selectively permit a first species, and substantiallyprevents any other species, present in the plasma to pass through the atleast one filter; a gas source connected to the cavity for supplying thegas to the cavity; a radiation source connected to the cavity forsupplying the radiation to the cavity; and a collection device incommunication with the plasma-opposing side of the at least one filter.34. The apparatus of claim 33, wherein the gas comprises hydrogen. 35.The apparatus of claim 33, further comprising at least one plasmacatalyst in the first cavity, wherein the at least one catalyst comprisea material that is at least electrically semi-conductive.
 36. Theapparatus of claim 35, wherein the material comprises at least one ofmetal, inorganic material, carbon, carbon-based alloy, carbon-basedcomposite, electrically conductive polymer, conductive siliconeelastomer, polymer nanocomposite, and organic-inorganic composite. 37.The apparatus of claim 36, wherein the material is in the form of atleast one of a nano-particle, a nano-tube, a powder, a dust, a flake, afiber, a sheet, a needle, a thread, a strand, a filament, a yarn, atwine, a shaving, a silver, a chip, a woven fabric, a tape, and awhisker.
 38. The apparatus of claim 37, wherein the plasma catalystcomprises carbon fiber.
 39. The apparatus of claim 38, wherein theplasma catalyst comprises a powder.
 40. The apparatus of claim 33,further comprising an active plasma catalyst comprising at least oneionizing particle in the cavity.
 41. The apparatus of claim 40, whereinthe at least one ionizing particle comprises a beam of particles. 42.The apparatus of claim 40, wherein the at least one ionizing particle isat least one of an x-ray particle, a gamma ray particle, an alphaparticle, a beta particle, a neutron, an electron, an ion, and a proton.43. The apparatus of claim 33, further comprising an ignition cavity forigniting a plasma with the electromagnetic radiation such that anignition plasma is formed therein, wherein the ignition cavity is influid communication with the first cavity, such that the ignition plasmacauses a first plasma to form in the first cavity.
 44. The apparatus ofclaim 33, wherein the filter comprises at least one of metal hydride,palladium, palladium oxide, ruthenium-palladium, palladium-silver,palladium-copper, uranium, magnesium, titanium, lithium-aluminum,lanthanum-nickel-aluminum, zirconium, ceramic, any combination thereof,and any alloy thereof.
 45. The apparatus of claim 33, further comprisinga voltage source electrically connected to the filter for applying abias to the filter.
 46. The apparatus of claim 33, wherein the filtercomprises a plurality of filtering layers.
 47. The apparatus of claim33, wherein the gas collection device at least includes a hydrogen fuelcell.
 48. The apparatus of claim 33 wherein the cavity is formed betweenan inner filtering tube and an outer metallic tube, the inner filteringtube comprising the filter.
 49. The apparatus of claim 48, furthercomprising a coaxial waveguide configured to direct the electromagneticradiation into the cavity.
 50. The apparatus of claim 48, wherein theinner tube has an outer diameter and the outer tube has an innerdiameter, wherein the ratio of the inner diameter to the outer diameteris between about 2.5 and about 3.0.
 51. The apparatus of claim 50,wherein the ratio is about 2.72.
 52. The apparatus of claim 48, whereinthe radiation comprises a TEM mode, and wherein the apparatus furthercomprises an electrically conductive endplate configured to at leastshift the mode axially by an odd number of quarter wavelengths.
 53. Theapparatus of claim 48, wherein the outer tube comprises a material thatsubstantially transmits the radiation.
 54. The apparatus of claim 48,wherein the main cavity has a first axial end that is configured toreceive the radiation.
 55. The apparatus of claim 48, further comprisinga voltage source configured to apply a bias to the filter.
 56. Theapparatus of claim 33, further comprising a channel configured to flow afluid such that the fluid is in contact with the filter.
 57. Theapparatus of claim 56, wherein the channel is formed in the filter.